t/lib-submodule-update.sh: clarify test

Keep the local branch name as the upstream branch name to avoid confusion.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: stefanbeller

t/lib-submodule-update.sh

@@ -664,8 +664,8 @@ test_submodule_recursing_with_args_common() {
 			cd submodule_update &&
 			git -C sub1 checkout -b keep_branch &&
 			git -C sub1 rev-parse HEAD >expect &&
-			git branch -t check-keep origin/modify_sub1 &&
-			$command check-keep &&
+			git branch -t modify_sub1 origin/modify_sub1 &&
+			$command modify_sub1 &&
 			test_superproject_content origin/modify_sub1 &&
 			test_submodule_content sub1 origin/modify_sub1 &&
 			git -C sub1 rev-parse keep_branch >actual &&